One of the basic ideas behind Bitcoin is that it exists
outside the control of any centralized power. That no longer seems
necessarily true. GHash.IO, a
particularly powerful organization of Bitcoin miners, turns out to have
at times controlled more than half of all Bitcoin mining activity—a
development that some think poses a new threat to the system.

The
creation of new Bitcoins has increasingly become the preserve of
sprawling groups known as mining pools. GHash.IO, the most successful of
these pools, theoretically has enough power to cheat the system. The
details are complicated, but it basically functions like this: The
Bitcoin system works only if users guarantee that coins aren't spent
twice. This requires a good deal of computational power, so the system
rewards users who verify blocks of transactions by awarding them new
Bitcoins. The process is known as mining. Just like miners looking for
gold, Bitcoin verifiers don't know exactly where the rewards will be,
but every once in a while, Bitcoin miners involved in verifying blocks
of transactions will unlock a reward. They announce it, and everyone
moves to the next block.

The supposed vulnerability comes when
miners don't tell everyone else when they've completed work on one
block. So-called selfish miners could theoretically keep their success a
secret to get a head start on the next block. This could allow them to
double-spend transactions, trading Bitcoins for dollars, erasing the
record of the transaction and getting their Bitcoins back.